Diseases of the retina, such as age-related macular degeneration (AMD), retinitis pigmentosa (RP), and other diseases, are the leading cause of severe visual impairment or blindness in the industrialized world. One hallmark of AMD, as in RP, is the degeneration and loss of cells of the retinal pigment epithelium (RPE). Bruch's membrane is also thought to be damaged; such damage may be the initiating stimulus for RPE demise. RPE cells are vital to the survival and proper functioning of retinal photoreceptors, which are the only cells in the eye which directly sense light. RPE degeneration in retinal diseases such as AMD and RP is related to the loss of photoreceptor function and the visual impairment that is associated with these diseases.
The RPE is located adjacent to the neural retina, directly opposed to the retinal photoreceptors. RPE cells in vivo form a one cell thick cobblestone-like tissue linked together by tight junctions, with differentiated apical and basal membranes. The RPE cells in vivo grow tightly packed together at high density to form a tight epithelium that acts as a barrier regulating transport between the photoreceptors and the underlying Bruch's membrane, choroid and the choroidal vasculature. The apical portion of the RPE is adapted to surround and engulf photoreceptor outer segments, in order for it to perform its vital functions of phagocytosis and digestion of shed photoreceptor tips, and of recycling retinal for re-use in photopigments. The basal portion of the RPE is apposed to Bruch's membrane, a highly vascularized supporting membrane which supplies the RPE and photoreceptors with needed oxygen and nutrients, and prevents the accumulation of carbon dioxide and other waste products which would otherwise impair retinal function. Damage to Bruch's membrane, which may occur due to accumulation of waste products from outer segment metabolism, for example, prevents the exchange of oxygen, growth factors and waste products. Such impaired exchange leads to hypoxia in the photoreceptors. In response, it is thought that survival signals are sent out to initiate the in-growth of neovascular vessels, and so to the wet form of AMD.
The iris pigment epithelium (IPE), which, like the RPE is derived from the neuroectoderm of the embryo, is located adjacent to the iris at the part of the eye opposite to the retina. Thus, in place in the intact eye, IPE cells are remote from retinal photoreceptors. Although much about IPE cell physiology and function remains unknown, like RPE cells, IPE cells in culture have been shown to be capable of phagocytosis of photoreceptor outer segments.
RPE cells may be grown on artificial substrates (Pfeffer, B. A., Chapter 10, “Improved Methodology for Cell Culture of Human and Monkey Retinal Pigment Epithelium,” Progress in Retinal Research, Vol. 10 (1991) Ed. Osborn, N., and Chader, J.; Lu et al., J. Biomater. Sci. Polymer Edn. 9:1187-1205 (1998), and Lu et al., Biomaterials 20:2351-2361 (1999). In addition, there have been attempts to use lens capsule tissue as a substrate for growing RPE and IPE cells (Hartman et al., Graefe's Archiv Clin Exp Ophthalmol 237:940-945 (1999); Nicolini et al., Acta Ophthalmol Scand October, 2000;78(5):527-31)).
Many approaches have been tried in the treatment of degenerative and progressive retinal diseases. For example, attempted treatments for AMD include photodynamic therapy, radiation therapy, and macular relocation in order to repair, retard the progression, or compensate for the effects of the disease. However, such approaches have not met with great success.
Since RPE cell loss occurs in many retinal diseases, the transplantation of cells has great attraction as a therapy and possible cure for AMD and other diseases. Direct transplantation of RPE cells into the retina has been attempted in order to replace lost RPE cells. However, this approach has not succeeded in the past, due in part to the failure of the transplanted cells to function properly and in part due to rejection of the cells by the host animals.
Transplantation of RPE cells has been suggested as a therapy for retinal dystrophy (U.S. Pat. No. 5,962,027 to Hughes and U.S. Pat. No. 6,045,791 to Liu). All patents named herein, both supra and infra, are hereby incorporated by reference in their entirety. In addition, experimental evidence that IPE cells could substitute for RPE cells has led to preliminary attempts to transplant IPE cells in animals and in order to ameliorate symptoms of AMD (Abe et al., Tohoku J. Exp. Med. 189:295-305 (1999), Abe et al., Cell Transplantation 8(5):501-10 (1999); Schraermeyer et al., Invest. Opth. Vis. Sci. 40(7):1545-56 (1999); Thumann et al., Transplantation 68(2)195-201 (1999); Abe et al., Tohoku J. Exp. Med. 191:7-20 (2000); Abe et al., Current Eye Research 20(4):268-275 (2000); Lappas et al., Graefes 's Arch Clin Exp Ophthalmol 238:631-641 (2000), Thumann, et al., Arch. Ophthalmol. 118:1350-1355 (2000)).
However, challenges to both IPE and RPE transplantation methods include i) difficulty in repairing the diseased Bruch's membrane, ii) inability to secure and position newly transplanted cells, and iii) lack of control over extracellular matrix signaling molecules that are critical to the structure, function, and survival of the pigment epithelial cell. For these and other reasons, techniques for IPE and RPE transplantation using antibiotics or immunosuppressants have not been successful. There has been no demonstration of significant visual improvement with these approaches, and problems of tissue reintegration remain. Thus, despite the apparent promise of the transplantation approach, AMD and other retinal diseases remain without successful therapeutic interventions.
Accordingly, there is need in the art for novel methods and apparatus for modification of tissues for transplantation and for transplantation of such tissues for the relief of retinal diseases.